A greenhouse by its very nature is required to be closed during cold periods to contain the heat to create its growing environment. At high humidity inside the greenhouse, water condenses on the inside of the greenhouse roof or cover when the temperature of the roof or cover is reduced to the dew point or lower. A greenhouse film made from an olefinic polymer has a surface characteristic of low surface tension and thereby is hydrophobic.
One characteristic of a thermoplastic greenhouse film in a humid greenhouse environment is that the water condensate forms on the surface of the film as free droplets. This condition is known as "fogging" and creates two basis problems for the grower. First, the transmission of sunlight, an essential requirement for plant growth, is reduced. Second, the free droplets coalesce to form larger droplets which fall onto and may damage the crop below. The damage to the crop may be to the extent that the market value based on crop quality is reduced, and in the case of a speciality crop such as flowers, the damage can render the crop unmarketable.
One way to overcome the above problem of fogging is to spray the surface of the film exposed in the greenhouse environment with a substance which when dry forms a coating which increases the surface tension of the film surface so as to reduce the interfacial tension between the surface and water condensate. A commercial product which gives the above described effect is that material sold under the trademark "Sunclear", manufactured by Solar Sunstill Company, San Antonio, N. Mex., U.S.A. The result is that the water vapour condensate wets the film with a clear sheet of water. The water then runs continually down the curvature of the film, instead of the water droplets "raining" down upon the crop below. The enhanced transmission of light increases the crop growth while preventing crop damage.
A second way to overcome the above problem of fogging is to incorporate a surfactant additive into the polymer prior to the polymer being converted into a film. The additive functions in this utility by migrating to the surface of the film. When droplets of condensation form upon this film, the surfactant dissolves in the droplet water and reduces the surface tension of the water to cause the droplet to spread and coalesce with other droplets until the film is covered with a sheet of water. The sheeted water thus allow drainage of condensate continually down the curvature of the film. Examples of commercial surfactants are Atmer 184.TM. and Atmer 103.TM. sorbitan stearate (ICI Specialty Chemicals).
Growers in the horticultural field have often expressed the need for a greenhouse film which can change its light diffusion properties according to the weather, i.e. the amount and nature of sunlight failing on the greenhouse film covering the greenhouse crops. Ideally, a grower would like a greenhouse film which is clear to impinging sunlight on temperate days but which darkens under the effect of high, direct sunshine in order to prevent excess heat build-up within the greenhouse. To-date, such a film is not available.
A shade paint commercially available under the trademark "VariShade" (Solar Sunstill Company, San Antonio, N. Mex., U.S.A.) is often applied to the outer surface of a clear greenhouse film. The paint on the film is transparent to direct sunlight when wet, for example, from rain, but which is cloudy or hazy when the film surface is dry under direct sunlight conditions. "Varishade II" shade paint is recommended for use on film surfaces facing outside and inside of the greenhouse and is, in addition, removable. However, applying such a shade paint coating to an erected greenhouse film is not satisfactory because of the economic cost, time and labour involved in its application, which, to be effective must be uniformly and properly applied.
A commercially available film having high haze and light diffusion properties made from a polyethylene resin sold under the trademark MARLEX D252 (Philips Petroleum Company) is used in the packaging of materials by reason of its suitable physical strength properties. The high haze characteristic of this commercial film is known to be due to the nature of its surface properties, which surface characteristic can be avoided, if desired, by suitable co-extrusion manufacture. However, the surface characteristic of this material in possessing haze properties is neither harmful nor beneficial for the packaging duty required of the film.
One physical characteristic considered of a polyethylene greenhouse film covering a greenhouse is the difference between the degree of its light diffusion and its clarity. While a diffusion cover may transmit essentially the same amount of light, diffusion results in the light at any location within the greenhouse to be less intense generally than those areas exposed to direct sunlight. Diffusion of the light spreads more of it to effect better exposed leaf area without also providing plants under dark shadows. Clear film allows direct light to be transmitted which, in strong sunlight conditions can be harsh to growing crops. On the other hand, under low light intensity conditions, such as under cloudy skies, or in low sunlight conditions of winter, late fall and early spring clarity of the film allows the maximum possible intensity of light passage through to the plants.
There is, therefore, a need for a satisfactory greenhouse film having satisfactory variable light diffusion properties, and which does not involve manual application after the greenhouse film has been erected over a greenhouse.